1. Field of the Invention
This invention relates to a logarithmic amplifier, and an image read-out apparatus provided with the logarithmic amplifier.
2. Description of the Prior Art
Techniques for reading out a recorded image in order to obtain an image signal, carrying out appropriate image processing on the image signal, and then reproducing a visible image by use of the processed image signal have heretofore been known in various fields. For example, as disclosed in Japanese Patent Publication No. 61(1986)-5193, an X-ray image is recorded on an X-ray film having a small gamma value chosen for the type of image processing to be carried out, the X-ray image is read out from the X-ray film and converted into an electric signal, and the electric signal (image signal) is processed and then used for reproducing the X-ray image as a visible image on a copy photograph or the like. In this manner, a visible image having good image quality with high contrast, high sharpness, high graininess, or the like can be reproduced.
Also, when certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored during its exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor. As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318, 4,387,428, and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation which has passed through an object such as a human body in order to store a radiation image of the object thereon, and is then scanned with stimulating rays, such as a laser beam, which causes it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. The image signal is then used to reproduce the radiation image of the object as a visible image on a recording material such as photographic film, on a display device such as a cathode ray tube (CRT), or the like.
Radiation image recording and reproducing systems which use stimulable phosphor sheets are advantageous over conventional radiography using silver halide photographic materials in that images can be recorded even when the energy intensity of the radiation to which the stimulable phosphor sheet is exposed varies over a wide range. More specifically, since the amount of light emitted by the stimulable phosphor sheet upon stimulation thereof varies over a wide range and is proportional to the amount of energy stored during its exposure to the radiation, it is possible to obtain an image having a desirable density regardless of the energy intensity of the radiation to which the stimulable phosphor sheet was exposed. In order to obtain the desired image density, an appropriate read-out gain is set when the emitted light is being detected and converted into an electric signal to be used in the reproduction of a visible image on a recording material or a display device.
In the radiation image recording and reproducing systems which use X-ray film or stimulable phosphor sheets, an analog output signal generated by a photoelectric detector, such as a photomultiplier, is fed into a logarithmic amplifier which logarithmically converts the analog signal. The logarithmically converted analog signal is sampled and converted into a digital image signal, which is made up of a series of image signal components corresponding to respective picture elements of the image. The purpose of logarithmically converting the signal generated by the photoelectric detector, such as a photomultiplier, is to compress the current of the signal output by the photomultiplier, which current varies over a very wide range of approximately -10.sup.-3 A to approximately -10.sup.-7 A, to a range in which the maximum to minimum current has a ratio of approximately 4:1. Thereby the processing of the signal is facilitated.
Most logarithmic amplifiers are constituted of an operational amplifier and a transistor which is used for logarithmic conversion and which exhibits excellent exponential function characteristics. Specifically, the transistor is located in the feedback circuit of the operational amplifier.
In a logarithmic amplifier constituted of an operational amplifier with a transistor in its feedback circuit, when a large current is fed into the logarithmic amplifier, a problem often occurs in that the effective resistance across the transistor decreases, and the logarithmic amplifier becomes unstable and oscillates. In order to eliminate this problem, a capacitor (a feedback capacitor) is often connected in parallel with the transistor.
In cases where the logarithmic conversion transistor and the feedback capacitor are connected in parallel, the operation of the logarithmic amplifier is stabilized. However, in such cases, when a small current is fed into the logarithmic amplifier, problems occurs in that the effective resistance across the transistor increases, and the frequency range of the logarithmic amplifier is limited by the effective resistance across the transistor and the capacitance of the feedback capacitor.
FIG. 4 is a graph showing examples of the frequency response characteristics of a logarithmic amplifier provided with a feedback capacitor. In the graph, the horizontal axis represents frequency, and the vertical axis represents gain. Each curve on the graph corresponds to a specific value of current which is fed into the logarithmic amplifier. The .times., .DELTA., O, .quadrature., and .cndot. marks respectively indicate a current of 0.1 .mu.A, 1.0 .mu.A, 10 .mu.A, 100 .mu.A, and 1 mA.
As shown in FIG. 4, when the current fed into the logarithmic amplifier is relatively small, the gain decreases rather sharply in the high frequency region.
In cases where the frequency response characteristics of the logarithmic amplifier are poor in the high frequency region, the problem described below occurs. Specifically, when the logarithmic amplifier is used in an image read-out apparatus wherein an image which has been recorded on X-ray film, a stimulable phosphor sheet, or the like is read out in order to obtain an image signal, the image reproduced from the thus obtained image signal exhibits low sharpness.
However, as described above, the signal which represents an image may occur over a range as wide as approximately 4 orders of ten (i.e. over a range of approximately -10.sup.-3 A to approximately -10.sup.-7 A). Therefore, the current fed into the logarithmic amplifier cannot be limited to values falling within a narrow range. That is, a large current may be fed into the logarithmic amplifier, and therefore a feedback capacitor is necessary in order to stabilize the operation of the logarithmic amplifier.